1. FIELD OF THE INVENTION
This invention relates to female receptacles at the ends of extension cords or wall outlets commonly used for suppling low wattage electrical power to household appliances and power tools.
2. DESCRIPTION OF THE PRIOR ART
A wide variety of receptacles are known to provide electrical contact between male electrical connectors having blade type terminals, commonly referred to as plugs, and female receptacles for receiving the blade type terminals, commonly referred to as outlets.
The most common household outlet has a pair of terminal contacts that receives the blade type terminal and are biased into contact with the blade terminal. The biasing force of the terminal contacts against the terminal blades is relatively light and the plug is easily inserted or removed from the outlet. Following installation, the terminal contacts remain in electrical connection with the power supply source.
A wide variety of other outlets designs satisfy, to varying degrees, the desire to more safely and securely connect a plug in an outlet. Again, in its basic form an electrical outlet has a pair of spring biased contact plates that receive the blade type terminals of the plug and frictionally hold the terminal in place against a relatively low withdrawal force. The low withdrawal force creates inconvenience where an inadvertent tug on an extension cord will pull the plug from the outlet. Continually powered outlets also pose the danger of electrical shock by accidental contact with the blades of the plug as it is inserted into the terminal or the contact of the live terminals with conductive material that was unknowingly placed into the outlet.
A number of the patents are directed to providing a safe outlet. U.S. Pat. No. 3,982,084 is directed solely to a safety receptacle and uses a nonconducting center prong to push internal electrical contacts into the conducting side prongs. U.S. Pat. No. 3,699,285 is similar in function and operation to the '084 patent and apart from making a good electrical connection is directed on to safety considerations of and outlet. The '285 patent uses a ground plug to act as a switch and close a circuit with the prongs as the prongs are inserted into the receptacle.
A number of other outlet arrangements will inhibit or prevent withdrawal of the plug from the outlet. U.S. Pat. No. 3,611,255 uses a locking ring to hold male and female receptacles together in a weather proof receptacle arrangement. U.S. Pat. No. 4,548,455 discloses a connector set with a relatively complex locking system having protrusions independent of the receptacle blades that are needed to achieve locking of the receptacle halves. U.S. Pat. Nos. 3,066,276 and 3,233,204 show the well known arrangement of arcuate prongs that are inserted into and rotated about a corresponding female receptacle to lock projecting portions of the prongs under retaining ledges defined by the receptacle. The '276 patent improves on the operation of the standard arcuate prong device by automatically rotating a portion of the female receptacle so that the male plug is locked into place by simple axial insertion of the plug. The '204 patent is similar in function to the '276 patent, but uses an internal camming device to automatically engage the prong projections with only a longitudinal movement of the male plug into the receptacle. U.S. Pat. Nos. 2,771,590 and 2,026,755 also use hooked or notched blades to furnish a locking function where the male plug is rotated relative to the female receptacle in order to lock or unlock the receptacles. Other arrangements that utilize hooked blades to lock receptacles are disclosed in U.S. Pat. Nos. 1,957,773 and 1,890,484. In the '773 and '484 patents, hook portions at the ends of the male connectors engage a cooperating ledge in a female receptacle and the plug is disengaged from the receptacle by displacing one or more of the hooks toward the center of the male plug.
It is also known to combine the locking action of a receptacle with a safe design to provide both functions in a single unit. U.S. Pat. No. 2,643,364 discloses a combination shockproof and secure electrical plug and receptacle set. In order to provide the locking function, the '364 patent employs special hooked blades that have a bottom projection to engage a ledge in the female receptacle.
One problem with the locking receptacles known to date is that they generally require specially designed terminals on the plugs. The prior art outlets can only provide the locking function when the blades of the plug are of a particular type that will cooperate with the female receptacle. These locking outlets will not positively engage the ordinary straight terminal blades of most plugs. Therefore locking outlets of the prior art offer limited utility for the average consumer.